"For example, in Cassandra such internal activities include the flush, also referred as minor compaction, which flushes in-memory data from the memtable to an S

"For example, in Cassandra such internal activities include the flush, also referred as minor compaction, which flushes in-memory data from the memtable to an SSTable file on disk, and may be accompanied by a substantial GC since a large chunk of memory becomes available for recycling." So these researchers don't unfortunately understand the very basics of GC (or C* for that matter). The basic findings of their paper is to be expected given the question they asked, but the specifics of their findings should be considered suspect given the clear lack of understanding of the technologies they're interrogating.